It is often desirable to add a data signal to an existing voice or television video communications channel in order to transmit both signals independently and simultaneously over the communications system.
Conventional systems in use today use out of band types of transmission with data transmission usually above the voice information but not in with the voice information. Many transmission lines are limited in bandwidth to just voice response because of loading coils or carrier channels and such systems cannot accommodate these out of band signals. A number of attempts have been made to transmit add-on data signals over a single transmission channel of predetermined bandwidth.
Attempts in the past to simultaneously transmit voice or video and data information simultaneously are represented by a number of issued patents. For example, U.S. Pat. No. 3,529,088 teaches a method of multiplexing sequential transmission of voice information and data information. Two patents, U.S. Pat. Nos. 3,894,190 and 4,238,849, teach a method of transmitting data over voice information by modulating digital message signals onto a subcarrier which lies in the ear psychological masking range (See FIGS. 3 and 2 of those references respectively) of a simultaneously transmitted broadcast message. U.S. Pat. No. 3,406,344 teaches a method of modulating the voice carrier to transmit low frequency signals.
U.S. Pat. No. 3,512,160 uses delay lines to transmit and detect transmitted signals which are decoded after the delay times to reconstruct the original signal(s). U.S. Pat. No. 3,838,444 teaches a method of altering a carrier signal which doubles the frequency of the carrier signal to indicate data transmission.
None of the foregoing systems achieves simultaneous real time transmission of data signals along with voice or other information with data signals occupying the same bandwidth as the other information signal and sufficiently dispersed throughout the bandwidth so that the data signals have no appreciable effect on the other information transmitted.